This disclosure relates to fluidized bed reactor systems and to the production of polycrystalline silicon from a thermally decomposable silicon compound such as, for example, trichlorosilane and, particularly, to methods that involve reduction of silicon deposits on reactor walls during polycrystalline silicon production.
Fluidized bed reactors are used to carry out multiphase reactions. In typical fluidized bed reactor systems a fluid is passed through a bed of granular material such as a catalyst or growing product particles. The flow of fluid causes the bed of granular material to become fluidized in the reactor.
Polycrystalline silicon is a vital raw material used to produce many commercial products including, for example, integrated circuits and photovoltaic (i.e., solar) cells. Polycrystalline silicon is often produced by a chemical vapor deposition mechanism in which silicon is deposited from a thermally decomposable silicon compound onto silicon particles in a fluidized bed reactor. The seed particles continuously grow in size until they exit the reactor as polycrystalline silicon product (i.e., “granular” polycrystalline silicon). Suitable decomposable silicon compounds include, for example, silane and halosilanes such as trichlorosilane.
In many fluidized bed reactor systems and especially in systems where material from the fluid phase chemically decomposes to form solid material such as in polycrystalline silicon production systems, solids may deposit onto the walls of the reactor. The wall deposits often alter the reactor geometry which can decrease reactor performance. Further, portions of the wall deposits can dislodge from the reactor wall and fall to the reactor bottom. Often the reactor system must be shut down to remove the dislodged deposits. To prevent an untimely reactor shut down, the deposits must be periodically etched from the reactor wall and the reactor must be cleaned thereby reducing the productivity of the reactor. The etching operations may cause stress to the reactor system due to thermal shock or differences in thermal expansion or contraction which may result in cracking of the reactor walls which requires the unit to be rebuilt. These problems are particularly acute in fluidized bed reactor systems used in the production of polycrystalline silicon.
Thus a continuing need exists for reactor systems and methods for producing polycrystalline silicon which limit or reduce the amount of deposits on the reactor walls.